The SCUBA 8-mJy survey - I. Submillimetre maps, sources and number counts

We present maps, source lists and derived number counts from the largest, unbiased, extragalactic submillimetre (submm) survey so far undertaken with the SCUBA camera on the James Clerk Maxwell Telescope (JCMT). Our maps are located in two regions of sky (ELAIS N2 and Lockman-Hole E) and cover 260arcmin2 , to a typical rms noise level of sigma 850 ~=2.5mJybeam-1 . We have reduced the data using both the standard JCMT surf procedures, and our own IDL -based pipeline which produces zero-footprint maps and noise images. The uncorrelated noise maps produced by the latter approach have enabled us to apply a maximum likelihood method to measure the statistical significance of each peak in our maps, leading to properly quantified errors on the flux density of all potential sources. We detect 19 sources with signal-to-noise ratios (S/N)>4, and 38 with S/N>3.5. To assess both the completeness of this survey and the impact of source confusion as a function of flux density, we have applied our source-extraction algorithm to a series of simulated images. The result is a new estimate of the submm source counts over the flux-density range S 850 ~=5-15mJy, which we compare with estimates derived by other workers, and with the predictions of a number of models. Our best estimate of the cumulative source count at S 850 >8mJy is per square degree. Assuming that the majority of sources lie at z >1.5, this result implies that the comoving number density of high-redshift galaxies forming stars at a rate in excess of 1000Msolar yr-1 is ~=10-5 Mpc-3 , with only a weak dependence on the precise redshift distribution. This number density corresponds to the number density of massive ellipticals with L >3-4L * in the present-day Universe , and is also the same as the comoving number density of comparably massive, passively evolving objects in the redshift band 1<z <2 inferred from recent surveys of extremely red objects. Thus the bright submm sources uncovered by this survey can plausibly account for the formation of all present-day massive ellipticals. Improved redshift constraints, and ultimately an improved measure of submm source clustering can refine or refute this picture.